This invention relates to pressure relief valves for fuel injection pumps of internal combustion engines and, more particularly, to a new and improved pressure relief valve of this type.
The use of an electromagnetic actuation device to control fuel pressure relief in a fuel injection device is described in German Offenlegungsschrift No. 35 11 492. As described therein, a pressure relief valve has an electromagnetic actuation device capable of positioning a valve-closing element into an open position which connects a fuel injection pressure chamber with a pressure relief outlet. That pressure relief valve includes a compression spring to restore the valve-closing element into a position in which the opening between the fuel pressure chamber and the fuel pressure relief outlet is closed. In order to minimize the force exerted by the actuation device in moving the valve-closing element into its open position in that valve, and also to minimize the movement necessary to restore the valve-closing element to its closed position, the valve seat is shaped so that, in the closed position, the valve-closing element has no surfaces which are subject to fuel pressure in the axial direction. However, immediately after the valve-closing element is separated from its valve seat, surfaces of the valve-closing element are exposed to fuel pressure in the axial direction so that the fuel pressure in the pressure chamber which is connected with a work chamber of the fuel pump is added to the electromagnetic force of the actuation device.
In a pressure relief valve of that type, a piezoelectric actuation device may also be used in place of the electromagnetic actuation device. Because of the reduction in fuel pressure in the valve pressure chamber, which is connected to the fuel injection valves of the internal combustion engine, a pressure relief valve of that type offers the advantageous possibility of ending the injection process before the work stroke of the fuel injection pump piston is complete. The closing of the pressure relief valve, that is, the movement of the valve-closing element into its closed position, is accomplished by a coordinated spring when the actuation device is de-energized after the end of the work stroke of the fuel injection pump piston, that is, after the end of the pump delivery stroke.